Cell searching is when a UE detects and identifies any base stations whose downlink signals are currently being received by the UE. Herein, “base station” or “BS”, “evolved node B” or “eNB”, and “cell” may be used interchangeably, depending on the context. Cell searching is an essential physical layer procedure for a UE because it ensures that the UE is able to synchronize in both time and frequency with an LTE downlink signal. Cell searching must be performed when the modem in a UE is powered up, and is required to support inter-frequency/intra-frequency and/or multi-Radio Access Technology (RAT) handovers/measurements while the modem is operating.
When cell searching, the UE relies upon certain signals to detect, identify, and initially synchronize with one or more eNBs. FIG. 1 shows some of these signals and their locations within an LTE Frequency Division Duplexing (FDD) downlink (DL) frame. FIG. 1 is based on a drawing in U.S. Pat. No. 9,078,146 to Gorokhov et al., which is incorporated herein by reference in its entirety. FIG. 1 shows that a single 10 ms DL radio frame includes ten 1 ms subframes, labelled 0 through 9, each subframe includes two slots, and each slot includes seven symbols (in the time domain). The symbols are labelled by each subframe (i.e., not by slot) 0 to 13, and each slot in the frame is labelled from 0 to 19. FIG. 1 does not show how the slots are also divided in the frequency domain by subcarriers.
Related to the present disclosure are the signals and channels labelled within slots 0, 1, 10, and 11 in FIG. 1. The eNB transmits a primary synchronization signal (PSS) in the last symbol of Slot 0 and Slot 10 and a secondary synchronization signal (SSS) in the second-to-last symbol of Slots 0 and 10. The eNB transmits cell-specific reference signals (CRSs), which identify the eNB and may also identify the antenna port of the eNB, in the first and fifth symbol of every slot (i.e., Symbols 9, 4, 7, and 11 of every subframe). Lastly, the eNB transmits the physical broadcast channel (PBCH) in the first through fourth symbols of the second slot (i.e., Symbols 7, 8, 9, and 10 of Slot 1 in Subframe 0).
In cell searching, the UE first detects the PSS, which is then used for a very rough timing synchronization (e.g., 5 ms), which allows the UE to then detect the SSS, which provides the physical layer cell identity NIDcell and allows the UE to synchronize with the frame. Since all of the frames in the network have the same timing, all of the nearby cells will have the same synchronization (discounting, for the moment, the complexities of path delay, for example). The UE can then decode the PBCH, which was scrambled prior to modulation with a cell-specific sequence that depends upon the cell's identity NIDcell and which contains the master information block (MIB).
The physical layer cell identity NIDcell is defined by Equation (1):NIDcell=3NID(1)+NID(2)  (1)where NID(1) represents the physical layer cell identity group and is provided by the SSS and NID(2) represents the physical layer identity within the physical layer cell identity group and is provided by the PSS. See, e.g., 3GPP TS 36.211, ver. 10.7.0 (2013-02), which is incorporated herein by reference in its entirety. The sequence used for the SSS is an interleaved concatenation of two length-31 binary sequences s0(m0) (n) and s1(m1) (n) (also known as “short codes”), where m0 and m1 are the indices for the short codes/sequences s0 and s1, respectively. Often (including in the instant disclosure), the indices are used to refer to the short codes themselves or the power measurements for the short codes. Together, short codes/sequences s0(m0) (n) and s1(m1) (n) provide 168 different physical layer cell identity groups, as shown in FIG. 2, which is taken from Table 6.11.2.1-1: Mapping between physical-layer cell-identity group NID(1) and the indices m0 and m1 in 3GPP TS 36.211, ver. 10.7.0 (2013-02).
Cell searches can result in false alarms (FAs), where non-existent cells (also known as false cells or ghost cells) are detected. One cause of this is when two nearby cells have SSSs which share a short code, i.e., either the same m0 or m1. This can be seen in FIG. 2, where, e.g., the cell ID NID(1)=1 has an m0=0, but so does NID(1)=30, NID(1)=59, NID(1)=87, NID(1)=114, NID(1)=160, and NID(1)=165. Accordingly, if a cell has NID(1)=30, and a nearby cell has NID(1)=114, they would have the same s0(m0) (n). This overlap in constituent short codes results in what is known as a “short code collision”, which, in turn, can result in a ghost cell being detected. Other causes of ghost cells include imperfect PSS/SSS cross-correlations between two cell IDs and random noise/interference.
Besides FAs, cell searches can also result in miss-detections (MDs), i.e., where actual, existing cells are not detected. To further complicate the matter, improvements in eliminating FAs usually result in the increase of MDs, so any engineering solution normally requires a tradeoff between the two.
FAs and MDs cause inaccurate cell search results, which, in turn, impairs modem performance, UE performance, BS performance, and overall cellular network performance. As one example, a UE reporting a ghost cell may trigger the network to initiate the Automatic Neighbor Relation (ANR) procedure, which is destined to fail (being initiated by a non-existent cell) and may further cause a radio link failure.
Traditional cell searches only report candidate cells based on a single-shot cell search followed by certain FA rejection procedures. The results suffer from the noisy nature of a single-shot cell search, and no consistency information is saved between consecutive searches.